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Casimir Effect with Quantized Charged Spinor Matter in Background Magnetic Field

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Casimir Effect with Quantized Charged Spinor Matter in Background Magnetic Field Casimir effect with quantized charged spinor matter in background magnetic field Yu. A. Sitenko1,2 1 Bogolyubov Institute for Theoretical Physics, National Academy of Sciences of Ukraine, 14-b Metrologichna Street, 03680 Kyiv, Ukraine 2 Institute for Theoretical Physics, University of Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland Abstract We study the influence of a background uniform magnetic field and boundary conditions on the vacuum of a quantized charged spinor matter field confined between two parallel neutral plates; the magnetic field is directed orthogonally to the plates. The admissible set of boundary conditions at the plates is determined by the requirement that the Dirac Hamiltonian operator be self-adjoint. It is shown that, in the case of a sufficiently strong magnetic field and a sufficiently large separation of the plates, the generalized Casimir force is repulsive, being independent of the choice of a boundary condition, as well as of arXiv:1411.2460v4 [hep-th] 5 May 2015 the distance between the plates. The detection of this effect seems to be feasible in the foreseeable future. PACS: 03.70.+k, 11.10.-z, 12.20.Ds Keywords: Casimir force, background magnetic field, boundary conditions, self-adjointness 1 1 Introduction Zero-point oscillations in the vacuum of quantized matter fields that are sub- ject to boundary conditions have been studied intensively over more than six decades since H.B.G.Casimir [1, 2] predicted a force between grounded metal plates, see reviews in [3, 4, 5]. The existence of this force is one of the few macroscopic manifestations of quantum theory, together with other remark- able phenomena such as superfluidity, superconductivity, kaon and neutrino oscillations, spectrum of black-body radiation. The Casimir force between material boundaries has now been measured quite accurately, it agrees with theoretical predictions, see, e.g., [6, 7], as well as other publications cited in [5], and this opens a way for various applications in modern nanotechnology. The Casimir force is closely related to the van der Waals force between 8 material bodies at such separation distances (> 10− m) that the retardation owing to the finiteness of the velocity of light becomes important. In view of this, it seems that the following two circumstances have to be clearly noted. The first one is that, as long as the intermolecular van der Waals forces are attractive, almost all experimental measurements reveal the attractive Casimir force; an evidence for the repulsive Casimir force has appeared just several years ago [8]. The second one is that, as long as the intermolecular van der Waals forces are due to electromagnetic fluctuations, the Casimir effect is caused by zero-point oscillations in the vacuum of the quantized electromagnetic field. The Casimir effect with other (nonelectromagnetic) quantized fields is mostly regarded as merely an academic exercise that could hardly be validated in laboratory. However, the nonelectromagnetic fields can be charged, and this opens a new prospect allowing one to consider the Casimir effect as that caused by zero-point oscillations in the vacuum of quantized charged matter fields in the presence of material boundaries and a background (classical) electromagnetic field inside the quantization volume. Whether the Casimir effect of this kind is attractive or repulsive – we shall get an answer in the present paper. Let us start by recalling that the effect of the background uniform electro- magnetic field alone on the vacuum of quantized charged matter was studied long ago, see [9, 10, 11, 12, 13] and review in [14]. The case of a background field filling the whole (infinite) space is hard to be regarded as realistic, whereas the case of a background field confined to the bounded quantiza- tion volume for charged matter looks much more plausible, it can even be regarded as realizable in laboratory. Moreover, there is no way to detect the 2 energy density which is induced in the vacuum in the first case, whereas the pressure from the vacuum onto the boundary, resulting in the second case, is in principle detectable. One may suggest intuitively that the pressure, at least in certain circumstances, is positive, i.e. directed from the inside to the outside of the quantization volume. A natural question is then, whether the pressure depends on a boundary condition imposed on the quantized charged matter field at the boundary? Thus, an issue of a choice of boundary conditions acquires a primary importance, requiring a thorough examination. It should be recalled that, in the conventional case of the Casimir effect with the quantized electromagnetic field, there exist physical motivations for different boundary conditions, for instance, corresponding to metallic or dielectric plates, see, e.g., [5]. Such motivations seem to be lacking for the case of quantized charged matter fields, but it was not distressing as long as this case, as we have already mentioned, was regarded as a purely academic one. Otherwise, in a situation which is supposed to be physically sensible, one should be guided by general principles, such as comprehensiveness and mathematical consistency, while seeking out boundary conditions. Namely, in the context of first-quantized theory, a quest is for the operator of a physical observable to be self-adjoint rather than Hermitian. This is stipulated by the mere fact that a multiple action is well defined for the self-adjoint operator only, allowing for the construction of functions of the operator, such as evolution, resolvent, zeta-function and heat kernel operators, with further implications upon second quantization. Whether the quest can be fulfilled successfully is in general determined by the Weyl – von Neumann theory of self-adjoint operators, see, e.g., [15, 16]. Thus, the requirement of the self-adjointness for the operator of one-particle energy (Dirac Hamiltonian operator in the case of quantized relativistic spinor fields) renders the most general set of boundary conditions, which may be further restricted by additional physical considerations. To avoid a misunderstanding, let us emphasize once more that quantized matter fields are assumed to be confined within the boundaries, and an is- sue of what is out of the boundaries is not touched upon. In a sense, this setup is the same as that in modeling hadrons as bags containing the quark matter, see [17, 18]. In distinction to the conventional setup for the Casimir effect, the impact of background fields on confined quantized matter fields is added along the lines discussed above. This generalization implies that the boundaries perceive an additional physical meaning, serving as a source of background fields which are inside the quantization volume. 3 In the present paper, we consider the Casimir effect in the generalized setup for a quantized charged spinor matter field in the background of an external uniform magnetic field; both the quantized and external fields are confined between two parallel plates, and the external field is orthogonal to the plates. It should be noted that a similar problem has been studied more than a decade ago [19, 20, 21] in a setup which is somewhat closer to the conventional setup for the Casimir effect. Namely, the authors of [19, 20, 21] assume that both the quantized and external fields are not confined between the plates but extend outside; the plates in their setup are regarded as the places where constraints on quantized fields are imposed, rather than as the real boundaries of the quantization volume. One of the purposes of the present paper is to compare the results obtained in these two different phys- ical situations. According to [19, 20, 21], there is no room for the validation of the aforementioned intuitive suggestion: the pressure in all circumstances is negative, i.e. the plates are attracted. On the contrary, by studying a response of the vacuum of the confined quantized spinor matter field on the external magnetic field with the strength lines terminating at the plates, I shall show that, in the case of a sufficiently strong magnetic field and a suffi- ciently large separation of the plates, the pressure from the vacuum onto the plates is positive, being independent of the choice of a boundary condition and even of the distance between the plates. In the next section we consider in general the problem of the self-adjointness for the Dirac Hamiltonian operator. In Section 3 we discuss the vacuum en- ergy which is induced by an external uniform magnetic field and compare the appropriate expressions for the cases of the unbounded quantization volume and the quantization volume bounded by two parallel plates. A condition determining the spectrum of wave number vector in the direction of the magnetic field is chosen in Section 4. Expressions for the Casimir energy and force are obtained in Section 5. The conclusions are drawn and discussed in Section 6. Some details of the derivation of results are given in Appendices A and B. 4 2 Self-adjointness of the Dirac Hamiltonian operator Defining a scalar product as 3 (ξ, χ)= d rξ†χ , ZΩ we get, using integration by parts, (ξ,Hχ)=(H†ξ, χ) i dσ ξ¯γχ, (1) − · ∂ZΩ 0 where ξ¯ = ξ†γ and 0 0 H = H† = iγ γ (∂ ieA)+ eA + γ m, (2) − · − 0 is the formal expression for the Dirac Hamiltonian operator in an exter- nal electromagnetic field (natural units ~ = c = 1 are used), ∂Ω is a two- dimensional surface bounding the three-dimensional spatial region Ω. Oper- ator H is Hermitian (or symmetric in mathematical parlance), (ξ,Hχ)=(H†ξ, χ), (3) if dσ ξ¯γχ =0. (4) · ∂ZΩ It is almost evident that the latter condition can be satisfied by imposing different boundary conditions for χ and ξ. But, a nontrivial task is to find a possibility that a boundary condition for ξ is the same as that for χ; then the domain of definition of H† (set of functions ξ) coincides with that of H (set of functions χ), and operator H is called self-adjoint.
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